A. Field of the Invention
This invention relates to a method for manufacturing a semiconductor device, which method includes a step of cutting a wafer with a dicing blade, and an exposure mask used in the same method.
B. Description of the Related Art
A step of manufacturing a semiconductor device may include a step of cutting a wafer with a dicing blade along scribe lines formed in a surface protection film. In this step, cutting is often done where no scribe lines are formed in an outer circumferential portion of the wafer. The step concerned with the cutting of the wafer will be described.
FIG. 16 is a main part plan view of reticle 51 for forming scribe lines. Reticle 51 is a reduced exposure mask, which has a plurality of device formation sections 52. The case where four device formation sections 52 are formed is shown in FIG. 16. Device formation sections 52 are separated from one another by scribe line pattern 53. Scribe line pattern 53 includes scribe lines 54 surrounding device formation sections 52 and scribe lines 54 arranged in a grid pattern and for separating adjacent ones of device formation sections 52 from each other.
FIG. 17 is a main part plan view of proximity exposure mask 57 for forming scribe lines. Proximity exposure mask 57 is a mask with which the whole region of wafer 1 can be exposed to light at one-time exposure so as to pattern a metal film, a surface protection film, etc. formed on wafer 58 (not-shown).
Thus, in a background-art exposure mask such as reticle 51 in FIG. 16 or proximity exposure mask 57 in FIG. 17, no protrusion portion 56 (designated by the broken line) of a scribe line is formed to stick out toward an outer circumference from intersection 55 of outermost peripheral scribe lines 54.
FIG. 18 is a plan view of scribe line pattern 62 when resist 61 on wafer 1 is exposed to light by use of reticle 51.
Wafer 1 is moved back and forth and left and right stepwise during exposure. Whenever wafer 1 is stopped, resist 61 of wafer 1 is exposed to light repeatedly. At one-time exposure on this occasion, resist 61 is exposed to light with a reduced pattern of reticle 51 so that the scribe line pattern of reticle 51 can be transferred to resist 61. When such exposure is repeated, scribe line pattern 62 is formed in the resist on wafer 1. Here, one-time exposure will be referred to as one shot. A pattern formed in resist 61 by this shot will be referred to as one-shot pattern 63. The number of shots for wafer 1 having a large diameter reaches about several ten. When each shot area is reduced, the number of shots reaches several hundred in order to form devices all over the surface of the wafer.
When, for example, reticle 51 is used, resist 61 is exposed to light to form scribe line pattern 62 in which adjacent ones of one-shot patterns 63 overlap each other in outermost peripheral scribe lines 64. Surface protection film 71 is etched by use of scribe line pattern 62 of resist 61 so that scribe line pattern 72 (not shown) is formed in surface protection film 71 (not-shown). Each region enclosed by the thick broken line designates an area of one-shot pattern 63. In FIG. 18, two one-shot patterns 63 formed in resist 61 are designated by the thick broken line. FIG. 18 shows a state in which two one-shot patterns 63 overlap each other in the outermost peripheral scribe lines 64 forming one-shot patterns 63. In order to increase the number of device formation sections 74, one-shot patterns 63 are formed to partially protrude from wafer 1 in the outer circumferential portion. In FIG. 18, one-shot patterns 63 protrude on the upper side, the right side and the lower left side.
In addition, each circle 66 in FIG. 18 designates a place where a crack (including chip) occurs easily when wafer 1 is cut along scribe line 73 formed in surface protection film 71. An arrow E indicates a cutting direction. Here, the term “chip” means a flaw such as a notch or a crack occurring in the range of from a dicing end surface to the inside of a die due to the dicing.
FIG. 19 is a plan view of scribe line pattern 62 when resist 61 on wafer 1 is exposed to light by use of proximity exposure mask 9. Scribe lines 64 are formed not to protrude from wafer 1. Also in this case, circle 66 in FIG. 19 indicates a place where a crack (including chip) occurs easily when wafer 1 is cut. In addition, an arrow E indicates a cutting direction.
FIG. 20 is a main part perspective view in which a portion F of scribe line pattern 62 formed in resist 61 shown in FIG. 18 or FIG. 19 is viewed from the direction of an arrow G. Each scribe line 64 is formed to be interposed between side walls of resist 61. Surface protection film 71 is exposed over scribe line 64.
FIG. 21 is a main part perspective view of scribe line pattern 72 of surface protection film 71 formed by use of scribe line pattern 62 of resist 61 in FIG. 18 or FIG. 19. Each scribe line 73 is interposed between side walls of surface protection film 71. Each of side walls 71a of surface protection film 71 is at a distance of about 10 μm from corresponding one of end portions 74a of device formation sections 74. In addition, the same metal film as metal film 75 formed in each device formation section 74 is often formed under surface protection film 71 outside the outermost peripheral scribe lines 73. The front surface (silicon surface) of wafer 1 is exposed over the scribe lines 73.
In addition, dicing blade 76 designated by the broken line moves along scribe line 73 in the direction of an arrow H from the far side toward the near side. Surface protection film 71 is normally made of polyimide etc. and metal film 75 made of an aluminum silicon alloy etc. is provided as an undercoat.
FIG. 22 is a main part perspective view showing a state in which wafer 1 is cut along scribe line 73 with dicing blade 76. FIG. 22 shows the state in which wafer 1 is cut along scribe line 73 with dicing blade 76 in such a manner that dicing blade 76 making turn 79 is moved in the direction of arrow 78 from the left side of scribe line 73 toward the right side thereof. Scribe lines 73 (partially designated by the broken lines) orthogonal to scribe lines 73 extending in the left/right direction are formed. Wafer 1 is cut along orthogonal scribe lines 73 from the far side toward the near side. Thus, dies are formed.
Scribe line pattern 72 of surface protection film 71 shown in FIG. 21 and formed by use of the resist mask shown in FIG. 18 has the following problem during the cutting. That is, crack 77 may often occur and become a defect in a device formation section 74 (a device formation section adjacent to outermost peripheral scribe lines 73) in the vicinity of intersection 80 corresponding to each circle 66 in FIG. 18. Dicing blade 76 enters from the left side and moves toward the right side while rotating as described above. In addition, dicing blade 76 moves from the far side toward the near side. The place where crack 77 has occurred is device formation section 74 disposed adjacently to the intersection 80 of the outermost peripheral scribe lines 73.
Next, a mechanism in which crack 77 occurs at the time of cutting wafer 1 will be described.
FIGS. 23A to 23C are views for inferring and explaining the mechanism in which crack 77 occurs at the time of cutting. FIG. 23A is a view in which cutting surface protection film 71 (including metal film 200) has been started. FIG. 23B is a view at a moment when a front end portion of dicing blade 76 reaches scribe line 73. FIG. 23C is a view in which dicing blade 76 is moving along scribe line 73. FIGS. 23A to 23C are views showing the vicinity of intersection 80 of the outermost peripheral scribe lines 73. Scribe lines 73 intersect each other in a T-shape.
In FIG. 23A to FIG. 23B, the front end portion of dicing blade 76 touches surface protection film 71 (also including metal film 200 as an undercoat) and moves while cutting surface protection film 71. On this occasion, micro vibration 81 occurs in dicing blade 76 moving while cutting surface protection film 71 (also including metal film 200 as the undercoat), as shown in FIG. 24. This vibration 81 is transmitted to surface protection film 71 and wafer 1 under surface protection film 71 (also including metal film 200 as the undercoat). Stress is applied to surface protection film 71 (also including metal film 200 as the undercoat) and wafer 1. When the stress increases, crack 77 (including chip) occurs in wafer 1.
In FIG. 23B to FIG. 23C, crack 77 occurring in wafer 1 is in progress due to the aforementioned stress. Since the moment of FIG. 23B, the front end portion of dicing blade 76 does not touch surface protection film 71. Accordingly, the aforementioned stress decreases as dicing blade 76 moves along scribe line 73. That is, as dicing blade 76 moves away from the place where dicing blade 76 has touched surface protection film 71, the aforementioned stress decreases so that extension of crack 77 decreases.
The aforementioned stress generated when dicing blade 76 touches surface protection film 71 almost disappears in a place ahead of about 100 μm from end portion 73a of scribe line 73 (the probability of occurrence of crack 77 in that place is in the order of about 0.1%). Accordingly, the width W of each scribe line 73 is set at about 100 μm.
FIG. 25 is a main part sectional view showing a state in which wafer 1 is cut with dicing blade 76 which moves to pass through surface protection film 71 and undercoat metal film 75 in an outer circumferential portion of wafer 1 where no scribe line 73 is formed. Surface protection film 71 and metal film 75 are located in a place to be cut with dicing blade 76. Dicing blade 76 touches surface protection film 71 and metal film 75 so that micro vibration 81 occurs in dicing blade 76. Crack 77 occurs in wafer 1 due to stress generated by vibration 81. This crack 77 extends to device formation section 74.
In addition, a reticle in which an exposure film is extended to an end portion of a main die has been described, for example, in FIG. 1 of JP-A-2-135343.
Moreover, JP-A-1-260451 has described that a dedicated reticle for scribe lines is used so as not to leave an unnecessary pattern in scribe lines in an outer circumferential portion of a wafer.
In addition, a method using a negative resist so as not to leave a conductive film in an outermost circumferential region has been described in JP-A-2002-246281.
As described above, a crack may occur with a certain probability even in a place where dicing blade 76 has moved over a distance of 100 μm along scribe line 73. Therefore, recently, there has been a strong demand to reduce the probability of occurrence of crack 77 (also including chip) to be lower than the current probability to improve the yield rate to thereby achieve reduction in the cost. Particularly in an automotive semiconductor element, it is necessary to reduce the percent defective extremely to the order of ppm or below. The following method is conceivable as a solution to the aforementioned problem.
FIG. 26 is a view showing a state in which wafer 1 is exposed to light up to its outer circumference by use of reticle 51. When scribe lines 73 are formed thus all over the outer circumferential portion, dicing blade 76 can cut wafer 1 without touching surface protection film 71 or undercoat metal film 75. Therefore, the probability of occurrence of a crack 77 can be reduced extremely.
However, an increase in the number of times of exposure (the number of shots) leads to lowering of the capability of an exposure apparatus. Thus, the processing time is elongated to increase the manufacturing cost.
In addition, when scribe lines 73 are formed up to the outer circumferential portion of wafer 1 as shown in FIG. 26, the probability that a die (which is originally a bad die) including a pattern defect which cannot be eliminated by visual inspection is determined as a good die to be assembled may increase so that concern remains in the reliability of a semiconductor device.
In addition, even when scribe line pattern 72 is formed in a surface protection film by use of the aforementioned reticle 51 or the aforementioned proximity exposure mask 57 according to the background art, there is a possibility that crack 77 (including chip) may be introduced into a device formation section 74 in an outer circumferential portion as soon as wafer 1 is cut. It is therefore essential to execute visual inspection. However, it is almost impossible to perfectly eliminate bad dies by visual inspection. For this reason, visual inspection is not performed on any device formation section in the outer circumferential portion of wafer 1 which is anticipated as a site where a bad die may occur, but all dies located in that site are regarded as bad. However, since there is a possibility that non-defective dies may be eliminated as defective dies in this method, there is a possibility that the yield rate may decrease to increase the manufacturing cost.